This invention relates generally to an optical fiber cable and, more specifically, to an optical fiber cable having a tensile reinforcement member.
Both single mode and multimode fibers may be assembled into cables. A typical single mode fiber may have a core diameter of about 2 .mu.m and a cladding diameter of about 70 .mu.m while a typical multimode may have a core diameter of about 60 .mu.m and a cladding diameter of about 70 .mu.m. Except for the case of liquid core fibers, core and cladding are generally made from materials having similar mechanical properties, and hence the mechanical properties of a single mode fiber will normally be similar to those of a multimode one made of the same core and cladding materials. Therefore, from the point of view of the mechanical considerations of cable making, there is little difference between the processing of both types when similar materials are used in their construction.
Glasses used for optical fiber manufacture include fused silica glasses, borosilicate glasses, and sode lime silicate glasses. Overall diameters of such fibers have evolved in part from optical requirements, but a limitation has been set by the brittle nature of the glass and the need to retain sufficient flexibility for incorporation in cables. Such fibers typically break at about 1-2% elongation when subjected to tensile stress, but behave elastically over most of the range of extension. This means that considerable stress can be applied without permanent deformation since the elastic moduli of glasses are high. However, as a result of their small cross-sectional area, the breaking tension of fibers is usually only of the order of a few hundred grams.
Some tensile reinforcement of individual optical fibers is therefore desirable merely to facilitate the laying up of a cable. This reinforcement may be provided by giving each fiber a plastic sheath. Such a sheath offers the possibility of further advantages, such as protecting the glass from chemical attack, and from damage by abrasion during winding and laying up operations. The sheath may also act to cushion the fiber from applied radial forces and give a measure of protection against the formation of kinks in the fiber of small radius of curvature. To take full advantage of some of these effects it would be desirable to use as thick a sheath as possible, but a compromise has to be reached in order to preserve adequate flexibility and to limit the total cross-sectional area of the cable for a given number of fibers.
When a system of parallel elements of uniform cross-section and equal length is extended, but not beyond the elastic limit of any element, the tension T developed is related to the strain S by the relationship:
T = S.SIGMA.EA
where E is Young's modulus of elasticity of an element and A is its cross-sectional area. In the case of a plastic sheathed glass fiber the equation becomes:
T = S(E.sub.1 A.sub.1 + E.sub.2 A.sub.2)
where the subscripts 1 and 2 refer to the glass and the plastic respectively. In this context it may be noted that it is immaterial whether the plastic is bonded to the glass provided that both materials are subjected to the same strain.
As a general rule, the elastic moduli of plastic materials are considerably lower than those of glasses, so that in order to make a significant contribution to the tensile strength the area A.sub.2 must substantially exceed A.sub.1. On the other hand, since the flexibility of a cylindrical rod is inversely proportional to EA.sup.2, it is advantageous to keep A.sub.2 as small as possible by using a plastic material with a large Young's modulus. Typically extruded plastic such as high density polyethylene (HDPE), polypropylene (PP), nylon, and polyethylene terephthalate (PETP) have moduli in the region of 150 to 200 kg/mm.sup.2, but some nylons extruded under ideal dry conditions can have a modulus as large as 300 kg/mm, though this is liable to degenerate to around 150 kg/mm in a moist atmosphere.
Assuming a modulus of 7,000 kg/mm for a glass it may be shown that for a 70 .mu.m diameter fiber the limit of 1% elongation is achieved at a tension of about 150 grams for a bare fiber. If, however, this fiber is sheathed in plastic material having a modulus of 300 kg/mm.sup.2 1% elongation occurs at a tension of 1 kg for a 0.6 mm external diameter sheath and at a tension of 2.5 kg for a 1.0 mm external diameter sheath. For a lower value modulus of 150 kg/mm.sup.2 1% elongation occurs at a tension of nearly 1.5 kg for a 1.0 mm external diameter sheath.
A tensile strength of the order of 1 kg is generally acceptable for the operations of cable making, such as bunching, stranding, braiding, sheathing, and armouring, but if the construction relies virtually exclusively on the sheathed fibers for its tensile strength, this strength is liable to be inadequate. In particular it is liable to be inadequate for withstanding the large force generally required to install cables in long ducts.
U.S. Pat. No. 3,883,218 to Slaughter teaches the disposing of sheathed optical fibers around a central tensile reinforcement member so that the strain on the fibers is reduced when the cable is stressed. The reinforcement member in the Slaugher patent is formed of steel and is therefore not as flexible as may be desired for some applications. Also, the fiber optic cable with a steel reinforcement member has a lower strength to weight ratio than desired for installation in ducts. Further, in certain military applications it is desirable to avoid the use of metal in a fiber optic cable. It is the purpose of the present invention to provide a fiber optic cable which overcomes the shortcomings of the above-described prior art cable, yet which still provides adequate protection of the optical fibers from undue stresses.